1. Field of the Invention
The present invention relates generally to light emitting devices, and, more particularly, to a system and method for the monolithic integration of a light emitting device and a heterojunction bipolar phototransistor (HPT) for extremely low bias voltage operation.
2. Related Art
Semiconductor light emitting devices in general and vertical cavity surface emitting lasers (VCSEL's) in particular are used for many applications including electronics, communication systems, and computer systems. Lasers produce light that can be transmitted directionally. In many applications of lasers, and particularly in many VCSEL applications, there is a need to precisely control the laser output power. The output power of semiconductor lasers is primarily determined by the bias current. However, it can be significantly altered by the ambient temperature and aging of the device. For this reason, control of the output power is realized by monitoring the laser output and adjusting the laser current to maintain a specified laser output power. The light measurement is typically performed using a semiconductor photodetector, a photoconductor, or a phototransistor as the light-detecting device, while the feedback loop is realized using an external electronic circuit. There are numerous implementations of such light-emitting/light-detecting device systems, and they differ in application and performance.
The two primary design issues relating to the laser/light-detector system are the cost of the device and the ability to provide performance required for a specific application. From a cost perspective, it is desirable to build the laser and the light detection device on the same chip using the same or similar fabrication technology. This is realized by monolithic integration of the laser and the light-detector. Monolithic integration implies that the individual laser and light-detector devices are completed jointly at the wafer level. From a performance perspective, there are a number of desired qualities. The light-detector current should track the directional light output from the laser, while minimizing the capture of the omni-directional spontaneous emission. The relationship between the directional laser output power and the light-detector current should be stable and repeatable. For proper operation the light-detector current should be within the range needed by the external analog feedback circuit. The existence of the light-detector and its biasing should have a negligible effect on the operation of, and in particular, the modulation properties of the laser. The laser modulation and biasing should have a negligible effect on the operation of the light-detector.
Lastly, the driver circuit must be considered. In computer communications applications the minimum bias voltage is an issue of increasing importance due to the desire to reduce computer power consumption. Today's computer architectures are using 3.3 volt (V) power supplies having a lower limit of approximately 3.1V. In the future and for other applications it is foreseeable that the power dissipation will be reduced even further requiring even lower bias voltage levels.
A preferable configuration of the laser/light-detector system is one in which the laser and light-detector are independently biased from the same power supply. In order to achieve this result, the power supply voltage must be larger than the laser operating voltage, which depends on the photon energy, and the light-detector operating voltage, which depends upon the light-detector structure, specifically, the photodetector reverse bias, or the phototransistor collector/emitter bias, required for efficient performance. For optical communications, the vertical cavity laser voltages range between approximately one to two volts. The typical photodetector reverse bias voltage is between 0.5 and 1 volt, while a phototransistor may operate with a collector/emitter bias of 1 to 1.5V, depending on the materials used. For other applications these voltages may vary.
An integrated laser and light-detector structure that enables independent biasing of the laser and the light-detector uses the lowest bias voltage. This is achievable by using a four terminal device structure in which two terminals are jointly connected to the power supply, thereby allowing arbitrary relative polarity between the laser and the light-detector. In three terminal monolithically integrated devices, the relative polarity between the laser and the light-detector is not arbitrary due to fabrication limitations.
In the past, light-detecting devices have been integrated with lasers with varying degrees of success. For example, some integration schemes use a photodetector and laser that have been independently fabricated on different chips. The two devices are integrated at the packaging stage, after fabrication, resulting in arbitrary relative polarity between the laser and photodetector. This integration scheme is referred to as "hybrid integration". The primary disadvantage of this approach is that the extra processing step of integrating the photodetector with the laser after fabrication undesirably adds manufacturing cost. Additionally, in many cases the relationship between the photodetector current and the laser output is neither stable nor repeatable, due to the fluctuation in the laser output beam shape.
Another scheme involves monolithic integration of a photodetector and laser where the coupling is realized using side emission, resulting in both three or four terminal devices. The main disadvantage of such devices is that the photodetector or phototransistor does not detect the directional laser output, but predominately captures the omni-directional spontaneous emission.
Finally, another scheme involves the monolithic integration of a laser and a photodetector or a phototransistor where the coupling is realized by top (or bottom) emission, resulting in both three and four terminal devices.
All of the implementations result in either three terminal devices where the laser and the photodetector share a common n-side (cathode) or a common p-side (anode), which as will be shown require a relatively high bias voltage for operation and in which the laser and the photodiode are electrically coupled; or where the laser and the phototransistor share a common ground terminal, which is not compatible with high-speed collector-driven (or drain-driven) electronic circuitry; or in four terminal devices, which are difficult and costly to fabricate.
Therefore, a monolithically-integrated three terminal device that can operate at an extremely low bias voltage, and enables electrical de-coupling between the laser and the photodetector is desired.
Disclosed in copending, commonly assigned U.S. patent application Ser. No. 09/167,961 is a laser and photodetector combination in which a photodiode is used as the laser output monitor device.